The present invention relates to disposable articles prepared using multi-line construction and especially to multi-line disposable diaper, sanitary napkin and bed pad constructions and to hot melt adhesives useful for the assembly thereof.
While a wide range of uses for hot melt adhesive compositions are known throughout the disposable industry, it has been found that a hot melt adhesive used for bonding in a particular use or application may be completely unsuitable for other uses or applications. Thus, various hot melt adhesive compositions have been proposed for use in the construction of disposable articles. Depending upon the type of construction employed, the adhesive must possess certain physical properties. Perhaps the most stringent properties are those required of adhesives to be used in the bonding of polyethylene films, or the like, to tissue or nonwoven substrates in the production of articles, particularly diapers, sanitary napkins and bed pads, using multi-line construction techniques. This class of disposable construction presents unique problems for the adhesive formulator. The adhesive must possess a high degree of adhesion since it is applied in the form of a number of very fine parallel longitudinal stripes thus requiring each line of adhesive to possess exceptionally high bonding properties. The adhesive must also possess sufficient adhesive and cohesive strength to provide high bond strength values when subjected to stress so the constructions cannot be easily separated. As an additional criteria, it is necessary that the adhesive, upon application, not absorb throughout the actual disposable construction and that the adhesive bonds not only remain secure but also be flexible even after the prolonged periods of storage. In addition to requiring heat and oxidation resistance on aging they must also possess sufficient bonding range and must be white or clear in color.
Initially hot melt adhesives based on ethylene vinyl acetate copolymers or on atactic polypropylene have been used for these multi-line constructions and neither of these approaches have resulted in adhesives possessing all the desirable properties discussed above. Thus, the polypropylene based adhesives have adequate adhesion requirements at the sacrifice of stability and bonding range while the ethylene vinyl acetate adhesives provide the flexibility while sacrificing adhesion and bond strength. Further, in order to formulate either type into adhesives which have the bonding range or open time required for these applications, the adhesive can be formulated as a semi-pressure sensitive adhesive which consequently has further reduced their cohesive strength resulting in loss of bonding during storage or transport at elevated temperatures.
Schmidt, U.S. Pat. No. 4,526,577 is primarily directed to hot melt adhesives based on linear A--B--A or A--B--A--B--A--B block copolymers. These adhesives contain the rubber copolymer, tackifier, oil and optional ingredients.
While the term "multi-line" construction is used herein to represent the above-described embodiment, it will be understood that the term may also be used to include articles constructed using a "multi-dot" or "multi-stripe" pattern or application, i.e. any assembly requiring the use of a large number of adhesive deposits, each deposit being of a small quantity of adhesive.
An object of the present invention is to provide a hot melt adhesive composition suitable for use in the construction of multi-line disposable articles. A further object is to provide hot melt pressure sensitive adhesive compositions which can be applied at relatively low temperatures that will retain superior bonding strength even when subjected to prolonged periods of storage.
This and other objects will be apparent from the description that follows.